Vol. 19 •Issue 2 • Page 17
Arterial Oxygenation in the Elderly
It’s Time to Look Forward
The 70-70 rule in respiratory care basically states that when individuals are 70 years old, their normal PaO2 should be about 70 mmHg. Those who have been in the profession for more than 20 years have probably learned that rule. What follows the natural progression of the 70-70 rule is the construct that at age 80, normal PaO2 would be 60 mmHg; and at age 90, a PaO2 of 50 mmHg would be reasonable. Fortunately, the guideline was generally not acceptable in persons over 90 years of age.
Considering the fact that in 2000, the fastest growing segments of the population (percentage wise) were those over age 85, considerable thought must be given to the logic of the 70-70 rule because current research disputes those findings. If the 70-70 rule is to be believed, then all nonagenarians are in need of supplemental oxygen. This leads right into an important reimbursement issue: Do all adults over the age of 90 then qualify automatically for home oxygen therapy?
Arterial blood gas (ABG) data are valuable only when compared to a normal reference range, and for the past five decades, dedicated researchers have conducted studies to determine normal lab values for different age ranges.
We need to focus on ABG reference ranges and the derived equations used to establish normal ABG values in the elderly so we can best meet the needs of this population in the years ahead.
In the process, we need to consider studies conducted from the 1960s to the present, look at potential reasons why discrepancies exist in the blood gas values considered “normal” for older adults and then consider the most current research on blood gas values in the elderly as we try to come to grips with normal values.
Arterial blood gas analysis is one of the most basic tests of pulmonary function. Done routinely on hospital wards, intensive care units, pulmonary clinics, emergency rooms, surgical suites and pulmonary function labs throughout the world, ABGs provide valuable information.
Improved Care for COPDers
In fact, those data are considered so important today that pulmonary function testing in and of itself is now being heavily promoted as an important intervention in reducing the escalating morbidity and mortality rate from COPD. Pulmonary screening, when made available and measured in those at risk for COPD, can provide evidence of pulmonary damage and can quantify the severity of the damage.
Diagnostic pulmonary function screening coupled with appropriately ordered ABGs can give physicians much of the information they need to prescribe interventional therapy. If physicians want to know whether their patient is a CO2 retainer, they order an ABG. If acid-base status is in question or if optimal oxygenation is not being achieved, blood gas values can drive the therapeutic intervention.
In ICUs, when caregivers work to liberate a patient from the ventilator, they order ABGs to confirm the patient’s readiness to wean. Pulmonary pathology, in any age group from neonates to centenarians, is managed more effectively when ventilation and respiration are adequate.
If you subscribe to the old adage “rules are made to be broken” (as a mother of three and grandmother of five, I tend not to sanction this notion), then careful consideration must be given to the 70-70 rule.
Based on information provided to health care providers in the 1960s and 1970s, a PaO2 of 70 mmHg at the age of 70 might have seemed reasonable. However, extrapolating the data to include the oldest-old (those from 80 to over 90 years of age) has proved the numbers are unreliable. The following chart lists some of the prediction equations available and what the PaO2 would be at age 90 if the equation were followed:
Although many of the equations come close to what is now presumed to be a normal PaO2 range for older adults, the difference between 49.7 and 82.7 is startling.
Challenges in Data
Determining normal blood gas values in adults over age 65 has been challenging for various reasons. For example, multiple chronic co-morbidities are common in the elderly, and lifestyle choices and environmental factors can impact the health of older adults. Researchers need to consider whether their subjects are current or former smokers or have over the years sustained environmental damage to their lungs as factors in their studies.
On the plus side, some older adults have maintained an active, healthy lifestyle well into their 80s and 90s and could be dubbed “super seniors.” Additionally, age-associated decremental changes in the lungs, although mean values of decline have been established, proceed at different rates in older individuals.
Inconsistencies in prediction formulas for blood gases over the past years were, in part, a result of the methods used to extrapolate values. In some cases, limited numbers of blood samples drawn from adults over age 70 were included in the research and were graphed and plotted; predicted values for those over 70 were determined by the extrapolated line.
Further adding to the inconsistencies in values was the practice of basing the research on convenience samples.
Ideally, reference values for ABG values should be based on a wide age range of subjects representative of the general healthy population.2 Because it is well known that body position affects ventilation and thus gas exchange, the patient position when the blood gas was drawn could also easily affect derivation of the prediction equation.
Gender-Related Surprises
Today, newer research indicates that age-related pulmonary changes leading to a decreased PaO2 are not as severe as previously thought. Although there does appear to be a decline up to age 75, after that, the PaO2 level of healthy non-smoking adults remains stable at about 83 mmHg.3
The most recent research, interestingly enough, is showing that a lower PaO2 may be more gender-related than age-related.
One paper published by Hardie and colleagues in Chest reported on a study done primarily to establish reference values for ABGs based on a general population appropriately representative of healthy elders between the ages of 70 to 100 years. All of the blood gases were drawn with the patients in the supine position.
Mean PaO2 and SaO2 were significantly higher and the mean P (A-a) O2 was lower in male as compared to female patients. Hardie sampled a relatively healthy population, and both non-smokers and ex-smokers were included in the sample. What he found was that the blood gas values were virtually identical in the never-smoker and ex-smoker groups.
Patients in that study also had spirometry measured as part of the research. The ex-smokers did tend to have a lower percentage of predicted FEV1 and FEV1/FVC ratios, however, the difference was not statistically significant.2
In another study, Hardie and colleagues tested the effect of body position on PaO2 and PaCO2 in “lung healthy” adults ranging in age from 67 to 88 years. Blood was drawn from the radial artery from patients in both seated and supine positions. The mean seated PaO2 was 78.9 mmHg, whereas the mean supine PaO2 was 73.9 mmHg.4
Although a 5 mmHg difference between a seated and supine PaO2 in a healthy older adult may be inconsequential, when evaluating a marginal patient for home oxygen therapy, the 5 mmHg difference may be critical.
Interestingly, the difference between the seated PaCO2 (37.95 mmHg) and supine PaCO2 (37.88 mmHg) was only 0.07 mmHg4, demonstrating again the relative stability of PaCO2 in older adults.
Some Final Thoughts
One final regression equation deserves mention, because barometric pressure can affect arterial blood gas results. In 1999, Crapo and colleagues did a study on ABG values of 339 adults ages 18-81 years at both sea level and at 1,400 meters. Their research indicated that all blood gas analytes correlated significantly with barometric pressure and that analytes associated with oxygenation (PaO2, SaO2 and P (A-a) O2) also correlated with age.
Based on their research, they derived the following regression equation:
Predicted PaO2 = 0.1834 PB – 0.24 (age) – 31.45 (at 1,400 meters).5
Although some contradictory data still exist among studies, it is important to note that researchers are actively including older adults in an attempt to establish more accurate reference ranges for ABGs.
The take-home message for respiratory therapists and respiratory therapy educators is simple: Optimal geriatric patient management requires us to stay current with new research, and new research soundly contradicts the 70-70 rule.
References
1. Statistical Abstract of the United States, 2000. Available at http://www.census.gov/prod/2001pubs/statab/sec01.pdf. Accessed 12/31/05.
2. Hardie JA, Vollmer WM, Buist S, et al. Reference values for arterial blood gases in the elderly. Chest (2004; 125: 2053-2060).
3. Beers MH (editor-in-chief). Aging and the lungs. Merck Manual of Geriatrics, 2000-2005. Chapter 75: Available at http://www.merck.com/mrkshared/mmg/home.jsp. Accessed 12/31/2005.
4. Hardie JA, M¿rkve O, Ellingsen I. Effect of body position on arterial tension in the elderly. Respiration (2002; 69:123-128).
5. Crapo RO, Jensen RL, Hegewald M, Tashkin DP. Arterial blood gas reference values for sea level and altitude of 1,400 meters. Am J of Resp and Critical Care Med. (1000; 160, 5: 1525-1531).
Helen Sorenson is a respiratory care educator at the University of Texas Health Science Center, San Antonio.
